CT-based planning of a single-radius femoral component in total knee arthroplasty using the ROBODOC system.

OBJECTIVE The purpose of this study was to propose our CT-based planning approach for knee arthroplasty on the basis of the femoral flexion-extension (FE) axis, and to evaluate whether this approach was valid by comparing the results with those obtained using conventional anterior-referenced planning. MATERIALS AND METHODS Virtual implantation of a sagittal single-radius component was performed in 34 normal knees. The transepicondylar axis (TEA) was identified as the FE axis, and was modified in the coronal plane to intersect the femoral mechanical axis at a right angle, if necessary. The implant was then selected that had a radius closest to the distance between the modified TEA and the distal condyle end. The implant position and size were compared for the two plans. RESULTS In almost all cases, slight modification of the TEA was required. However, there was no significant change in the distance from the TEA to the posterior and distal condyles. In comparison to the results obtained with our planning approach, the conventional plan resulted in antero-superior deviation of the implant sagittal center to the FE axis and/or selection of a larger size of prosthesis. CONCLUSION Although the TEA must be modified slightly when referencing it as the FE axis, our planning approach may be valid for femoral single-radius components because the single axis of the component could be matched with the FE axis.